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Planetary and Space Science 54 (2006) 1211–1224
www.elsevier.com/locate/pss
Initial interpretation of Titan plasma interaction as observed by
the Cassini plasma spectrometer: Comparisons with Voyager 1
R.E. Hartlea,, E.C. Sittlera, F.M. Neubauerb, R.E. Johnsonc, H.T. Smithc, F. Craryd,
D.J. McComasd, D.T. Youngd, A.J. Coatese, D. Simpsona, S. Boltonf, D. Reisenfeldg,
K. Szegoh, J.J. Berthelieri, A. Rymere, J. Vilppolaj, J.T. Steinbergk, N. Andrel
a
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
b
University of Koln, D 50923 Koln, Germany
c
University of Virginia, Charlottesville, VA 22904, USA
d
Southwest Research Institute, San Antonio, TX 78228-0510, USA
e
Mullard Space Flight Center, Dorking, Surrey RH5 6NT, UK
f
Jet Propulsion Laboratory, Pasadena, CA 91109, USA
g
University of Montana, Missoula, MT 59812, USA
h
KFKI-RMKI, KFKI Research Institute for Particle and Nuclear Physics, Budapest H-1525, Hungary
i
Centre d’etude des Environnements Terrestre et Planetaires, St. maur-des-Fosses, 94107 France
j
University of Oulu, Linnanmaa, FIN-90014, Finland
k
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
l
CESR, Toulouse, 4346 31028, France
Received 10 November 2005; received in revised form 21 November 2005; accepted 4 May 2006
Available online 14 August 2006
Abstract
The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan’s plasma environment when the Cassini Orbiter
flew through the moon’s plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will
be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate
previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma
wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn’s local noon. These
similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following:
(A) The light and heavy ions, H+and N+/O+, were observed by PLS in Saturn’s magnetosphere in the vicinity of Titan while the higher
+
+
mass resolution of CAPS yielded H+ and H+
2 as the light constituents and O /CH4 as the heavy ions. (B) Finite gyroradius effects were
+
apparent in PLS and CAPS measurements of ambient O ions as a result of their absorption by Titan’s extended atmosphere. (C) The
+
+
+
principal pickup ions inferred from both PLS and CAPS measurements are H+, H+
2 , N , CH4 and N2 . (D) The inference that heavy
pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing
down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan.
(F) Strong mass loading just outside the ionotail by a heavy ion such as N+
2 is apparent in PLS and CAPS measurements. (G) Except for
the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures
observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that
observed by CAPS.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Titan; Plasma; Pickup ions; Mass loading; Composition; Cassini
Corresponding author. Tel.: +1 301 2868234; fax: +1 301 2861663.
E-mail address: [email protected] (R.E. Hartle).
0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pss.2006.05.029
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1. Introduction
The complex interaction of Saturn’s outer magnetosphere with Titan’s atmosphere was observed for the first
time by plasma and field instruments when Voyager 1 flew
by Titan on November 12, 1980. The initial analysis of the
plasma science instrument (PLS) ion and electron spectra
by Bridge et al. (1981) found that the rotating magnetosphere was in a subsonic state as it was deflected around
Titan, forming a wake through which Voyager 1 passed.
The magnetic field measurements by Ness et al. (1981)
revealed that Titan does not have an intrinsic magnetic
field, but has an induced magnetosphere with a bipolar tail
in the wake region. These initial results were followed by
the more comprehensive analysis of Hartle et al. (1982,
referred to as Paper I), Ness et al. (1982) and Neubauer
et al. (1984). Following these early analyses and interpretations, several atmosphere, ionosphere and interaction
models (Yung et al., 1984; Yung, 1987; Toublanc et al.,
1995; Keller et al., 1998; Ledvina and Cravens, 1998;
Brecht et al., 2000) were developed. These models
stimulated Sittler et al. (2004, 2005, latter paper referred
to as Paper II) to revisit the Voyager 1 data, producing the
following: (1) Pickup ions H+ and H+
2 dominate at the
largest distances from Titan’s magnetic tail, followed by
+
CH+
4 at intermediate distances and N2 just outside the
tail. The relative abundance of these ions is consistent with
the densities of the corresponding neutral exosphere
sources. (2) Exospheric CH4 and pickup ion CH+
4 are
sources of carbon in Saturn’s magnetosphere. (3) Finite
gyroradius effects play an essential role in the removal of
background magnetosphere plasma by Titan’s upper
atmosphere. (4) The finite gyroradius effects also implied
that the observed hot keV ion component of the ambient
plasma is a heavy ion such as N+/O+. (5) A minimum
‘‘ionopause’’ altitude of 4800 km at the subflow point was
estimated by a new mass loading approach.
The Voyager 1 and Cassini TA flyby trajectories at Titan
are very similar so that direct comparisons of the PLS and
Cassini plasma spectrometer (CAPS) plasma measurements are feasible. One similarity is that each flyby
trajectory passed through the wake produced by Saturn’s
rotating magnetosphere plasma as it flows past Titan.
Another is the similarity of the solar zenith angles at each
flyby, because the TA encounter was 10.6 h LT relative to
Saturn and Voyager 1 was 13 h LT. The Cassini flyby
trajectory in Titan-centered coordinates is shown in Fig. 1,
where the y-axis points toward Saturn, the x-axis is in the
corotational direction of the rotating magnetosphere and
the z-axis is parallel to Saturn’s rotational axis. The Cassini
trajectory is super-imposed on the model interaction
originally derived by Hartle et al. (1982) from the Voyager
1 data. The ‘‘views’’ in Fig. 1a and b, in the –z and –x
directions, respectively, show the similarities of the
Voyager 1 trajectory to that of the TA encounter, with
the exceptions that Voyager 1 is further down the wake and
has an almost equatorial pass relative to Cassini’s mid-
Fig. 1. Geometrical properties of Titan interaction projected onto orbital
plane (x,y). Spacing on x- and y-axes are in intervals of 103 km. On the
Voyager 1 trajectory, the numbers are positions where PLS ion
measurements were made, while the CAPS IMS measurements were made
almost continuously along the Cassini trajectory. The Voyager 1 trajectory
is inclined 101 to the x–y plane, while TA is inclined 17.31. Voyager 1
crosses the x–y plane from above at 0542 UT. TA does not cross the x–y
plane in the ionotail; however, for orientation, it crossed the terminator
plane above the equatorial plane at 15:29:59 UT. Voyager 1 model
exobase and ionopause boundaries from Paper I are shown along with
streamline flow and an H+ pickup ion trajectory (to scale). The inset in the
upper right shows the directions of the normal vectors to the four PLS
Faraday cups.
latitude pass. For TA, the closest approach altitude was
1764 km from the surface at 15:30:08 SCET (spacecraft
event time in [h:min:s] throughout). Voyager 1’s closest
approach was 4400 km from the surface at 05:40:20
SCET. The model describing the Voyager 1 interaction
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shows the neutral exobase, ionopause boundary and the
incoming flow, deflected 201 from the corotational direction. The inset in Fig. 1a shows pointing directions of the
A, B, C, and D Faraday cups of the PLS on Voyager 1.
The CAPS instrument (Young et al., 2004) is composed
of an ion mass spectrometer (IMS), electron spectrometer
(ELS) and an ion beam spectrometer (IBS). We will
primarily be discussing data from the IMS and ELS. The
IBS is most important for Titan’s ionosphere. The IMS
covers the ion energy-per-charge range 1 VpE/Qp50 kV
and the ELS has the electron energy range 1 eVpEp28 keV. Both instruments take simultaneous measurements in
collimators divided into eight angular sectors separated by
1213
201 in the collimator plane. The IMS takes singles data
(E/Q spectra), coincident ion data for pre-selected ion
species and coincident time-of-flight (TOF) data (or Bcycle data, E/Q vs. TOF) used for detailed compositional
analysis. An IMS energy sweep takes 4 s, the ELS sweep
takes 2 s and the B-cycle takes 256 s. For the B-cycle data,
we sum 64 energy sweeps and collapse all eight angular
sectors for high sensitivity. An actuator sweeps the field of
view of the sensors to optimize pitch angle coverage. The
actuator was scanned at a rate of 11 s1 over a range of
+1071 to 811 during Titan-A, with a sweep across the
ram direction for the immediate Titan encounter. The
pointing directions of the eight collimators in the IMS
Fig. 2. IMS collimator frame orientation variation as the instruments actuator moves in window washer mode between 71041 of actuator angle.
Figs. 2a, b and c correspond to times 13:35:36, 15:14:48 and 17:35:04 SCET. See text for details.
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collimator plane for three scan angles of 1041, 01 and
1041 (note: this represents the nominal scan range that
differs from the actual scan used above) are shown at
13:35:36, 15:14:48 and 17:35:04 SCET in Figs. 2a, b and c,
respectively. As will become evident below, the earliest and
latest time corresponds to regions close to and on either
side of the plasma wake, but not in the region where mass
loading occurs. The mid-time occurs in the mass loading
region described below. Note that the coordinates in Fig. 2
differ from those in Fig. 1, where the x- and y-axes of the
former are rotated 901 counter clockwise about the z-axis,
which points in the same direction in both figures. The red
arrows show that the x-axis in Fig. 2 points toward Saturn
while the nominal corotation vector is in the –y-direction.
2. Ion spectra, flow and composition
Fig. 3 shows Voyager 1 PLS ion distribution functions,
F, vs. energy per charge, E/Q, observed in the A, B, C and
D Faraday cups at the numbered observation points on the
trajectory in Fig. 1. We use Figs. 1 and 3 to give a preCassini overview of plasma flow and composition. Simulations of spectra, described in Paper I, revealed background
magnetosphere ions H+, N+ and/or O+ as indicated.
Since the latter two ions were not distinguishable, they
were expressed as N+/O+ in Paper I. Ions first identified in
Paper I are shown in black and those identified in Paper II
are in red. Simulations of pickup ion ring distributions
derived from model exosphere densities in Papers I and II
+
+
+
led to the identification of H+, H+
2 , N , CH4 and N2
ions as shown. As Voyager 1 approached the exobase of
Titan, the background N+/O+ density decreased considerably because the large N+/O+ gyroradii resulted in their
penetration of Titan’s atmosphere, where they were
‘‘absorbed’’ and lost to the flow. This finite gyroradius
effect described in detail in Paper II did not seem to affect
H+ with its smaller gyroradius. The ion simulations also
revealed an increased reduction in the flow velocity as
Voyager 1 approached the magnetotail (panels 3–4),
slowing to speeds of 5–10 km s1 at measurement point 4.
The slowing down at point 4 was suggested in Paper I to be
due to pickup of the heavy 28 amu ion thought to be N+
2 .
However, a great deal of the slowing down was attributed
in Paper II to mass loading by CH+
4 , because its parent,
CH4, is the dominant neutral exosphere constituent over
most of the slowing down region (see below).
These Voyager I PLS observations are now compared
with those made by Cassini CAPS, an instrument with a
higher time resolution, covers a wider energy range and a
TOF capability to identify species mass. We begin with the
CAPS E/Q vs. time spectrogram of IMS ‘‘singles’’, shown
in Fig. 4, with all eight angular sectors (note: the striated
appearance is a consequence of the scan motion of the
actuator). The spectra are termed singles because ion rates
are non-coincident and consequently contain no composition information. This spectra has been used in differing
formats by Szego et al. (2005), Crary et al. (in review;
referred to as Paper III) and Hartle et al. (2006, referred to
as Paper IV). All three investigations used time segments of
the data differing from Fig. 4 and the first two summed
over all angular sectors. The background plasma outside
the Titan interaction region has been analyzed by Szego et
al. (2005) and Crary et al. (Paper III)and. They find that
the E/Q range of plasma outside the Titan interaction
region shows up as two peaks with E/Q 200 V and E/Q
+
+
+
+
2 kV, consistent with H+, H+
2 , N / CH2 and O /CH4 .
In Paper III, Crary et al. found the ions to be flowing at a
subrotation speed 110720 km s1 (corotation speed
200 km s1) and identified the ion masses above from
ion counts vs. TOF data. These composition results
Fig. 3. Ion distribution functions, F, vs. energy (eV)/charge observed by PLS, where each numbered column corresponds to numbered observation points
on trajectory in Fig. 1. A, B, C and D refer to the sensor cups and the slanted lines are the instrument noise levels. Ions are placed above the portions of the
distributions they populate, where those identified in Paper I are in black and those of Paper II are in red.
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Fig. 4. IMS singles spectrogram observed on TA trajectory. All eight angular sectors are shown as energy per charge, E/Q (eV) vs. SCET (in [h:min:s]
throughout) vs. counts depicted by color scale in logarithmic counts to right. Discrete energy scans appear as vertical swaths. The white vertical lines are
data gaps.
obtained on the anti-Saturn and Saturn facing sides are
consistent with the Voyager 1 measurements except that
the high data rate of CAPS made it possible to identify
+
H+
2 in addition to H , and distinguish the 14 amu ion,
+
+
N / CH2 , from the 16 amu ion, O+/CH+
4 .
An additional analysis of background magnetosphere ion
composition on the ingress and egress legs of TA are made
using CAPS E/Q vs. TOF spectrograms, shown in Figs. 5a
and b, respectively. The ingress spectrum is from 12:00 to
14:00 SCET and the egress from 17:00 to 19:00 SCET, both
in the vicinity of Titan, but well away from the mass loading
regions. During these long observation intervals, counts
were integrated over many B cycles. Spectral ‘‘lines’’ for ion
masses 1, 2 and 16 amu are identified and interpreted to be
+
H+, H+
2 and O . This ambient plasma is hot, having energy
ranges of 0.016–6.3 keV for H+, 0.1–4 keV for H+
2 and
1–4 keV for O+. The identification of H+ and O+ is
strengthened by the appearance of H and O lines, their
instrumental ‘‘fingerprints’’ (for interpretation of negative
ions, see Young et al., 2004). The 16 amu TOF channel may
also contain CH+
4 and the 14 amu TOF channel may signal
the presence of N+/CH+
2 , all of which were more apparent
in the counts vs. TOF analysis in Paper III as described
above. Future analysis may clarify such uncertainties in the
E/Q vs. TOF spectrograms. In Fig. 4, the effects of mass
loading are depicted by the slowing down of plasma over the
period 15:00 to 15:18 SCET. The greatest ion counts
from 13:00 to 15:18 SCET appear in the three lowest sectors
in Fig. 4, sectors 6 through 8. These sectors correspond
to the sector pointing directions of the same numbers in
Figs. 2a and b, which are the ones pointing most closely into
the corotation direction of the magnetosphere. In the mass
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from the main flow; however, they are consistent with
velocities expected of pickup ions traveling in the subrotating plasma depicted in Figs. 1 and 9 below. These signatures
are consistent with those expected from narrow beam-like
fluxes of heavy pickup ions such as CH+
4 , as will be
discussed below.
In the mass loading region, the E/Q range in Fig. 4
decreased from an initial range 0.2–2 kV to 5–50 V,
which corresponds to speeds 5–15 km s1 for N+
2 , an ion
inferred from Voyager 1 PLS measurements (Papers I and
II) and suggested by the CAPS TOF data discussed below.
This mass loading region, on the anti-Saturn side of Titan,
was first identified in Paper I and ascribed to the addition
of pickup ion mass by the flowing plasma and momentum
exchange between the flowing plasma and pickup ions. The
presence of mass loading on the anti-Saturn side of Titan
was also discussed briefly in Papers III and IV. At the time
of Paper I, only H+ and N+
2 were thought to be the pickup
ions. However, in Paper II, the revised model neutral
exosphere, shown in Fig. 6, was used for source calculations of pickup ions. New exosphere species not included in
Paper I are H2, CH4 and suprathermal N* (Yung et al.,
1984; Yung, 1987; Toublanc et al., 1995; Keller et al., 1998;
Michael et al., 2004). They were added in Paper II,
including the corresponding ionization rates, to study mass
+
+
loading due to the five pickup ions H+, H+
2 , N , CH4 and
+
N2 . Since then, in situ measurements of the Titan’s neutral
atmosphere have been made by Waite et al. (2005) using
the Cassini ion neutral mass spectrometer (INMS). The
exobase densities used in Fig. 6 (Papers I and II) for N2 and
CH4 were 108 cm3 and 4 107 cm3 while those observed
by the INMS are 3 107 cm3 and 2.5 106 cm3,
respectively. The density differences in N2 may be due to
exobase temperature differences, where those used in
Papers I and II were 160 K (obtained from Broadfoot
et al., 1981) while those inferred from the INMS were
Fig. 5. (a) Background or ambient magnetosphere plasma observed on
incoming leg of TA by IMS. Coincident TOF color spectrogram shown as
function of ion energy per charge, E/Q (eV), vs. TOF channels with ion
counts depicted by the color legend in logarithmic counts to the right. The
total number of counts for a given E/Q are obtained by multiplying the
legend by 225, corresponding to the counts from all B cycles between 12:00
and 14:00 SCET. (b) Same as (a) except being for outgoing leg between
17:00 and 19:00 SCET.
loading region, a few ‘‘bright spots’’ (identified by white
arrows) near a kV appear in sectors 1 through 4 of Fig. 4,
corresponding to pointing directions 1–4 in Fig. 2b. The ions
producing these counts have velocity components well away
Fig. 6. Titan model neutral exosphere densities vs. height above exobase.
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149 K, the atmosphere being cooler and less extended. The
differences in the CH4 densities are much larger and are
probably not just due to temperature differences in the
atmosphere. We note that the exobase density for CH4
used in Paper II was derived from a model (see Yung et al.,
1984; Yung, 1987) in contrast to the one derived from an
actual measurement by INMS.
From 15:22 to 15:40 SCET, the Cassini spacecraft was
within Titan’s ionotail and passed through the ionosphere,
where a vast range of hydrocarbon ions have been
identified by the IBS instrument (described in Paper III).
A spacecraft maneuver started around closest approach
and ended 16:40 SCET, which changed the pointing of
CAPS as indicated by sector pointing in Fig. 2c. The largest
counts come from sectors 1 through 4 of Fig. 4,
corresponding to sector pointing directions 1 through 4
in the sensor alignments shown in Fig. 2c. As on the
inbound leg, these sectors also point approximately into
the corotational flow direction. Fig. 4 shows that after
15:40 SCET, the spacecraft exited the ionosphere and
quickly returned to the dynamical conditions of Saturn’s
magnetosphere plasma. This was also the case on Voyager
1’s outbound leg (Paper I) as one can see in Fig. 3. That is,
in contrast to that on the anti-Saturn side, the rapid
recovery is due to the lack of sufficient pickup ions to mass
load the plasma. This suggests that the paucity of pickup
ions on the Saturn facing side occurs because the cycloidal
trajectories of the heavy ions intercept Titan’s atmosphere,
where most of them are captured. The Cassini magnetometer observed boundaries of the induced magnetosphere
between 15:10 and 15:40 SCET (Backes et al., 2005).
These boundaries lie between the mid-mass loading region
on the anti-Saturn side and just beyond the ionosphere on
the Saturn facing side of Titan.
3. Magnetosphere ion erosion by Titan
After further analysis of the PLS ion spectra shown in
Fig. 3, it was pointed out in Paper II that, as Voyager 1
approached Titan, the high-energy part of the spectrum
disappeared first, which was identified as the heavy
component N+/O+ in Paper I. Then the lower-energy
component, H+, disappeared when the spacecraft was
close to the ionopause of Titan. On exiting the magnetotail,
the light ion component recovered first followed by the
heavy, more energetic component. These variations on
both sides of Titan were shown to be finite gyroradius
effects in Paper II, where ambient H+ and N+/O+ are
absorbed by the atmosphere on those terminator streamlines whose closest approach distances to the exobase fall
inside the ambient gyrodiameters, 800 km and 11,200 km,
respectively. Although similar erosion occurs on both antiSaturn and Saturn facing sides, the combination of the
counterclockwise ion gyromotion (Fig. 1 view) and the
upstream field of view of PLS and CAPS results in differing
signatures on each side of Titan. That is, background O+
should be observed beyond an ion gyrodiameter from the
1217
exobase on the anti-Saturn side and show no signs of
absorption. As one moves inside this distance and the ion
orbits cross the exobase, O+ is expected to diminish as the
atmosphere erodes it. On the Saturn facing side of Titan,
the same spatial structure of O+ would be expected if PLS
and CAPS had downstream fields of view. Instead, with the
instruments facing upstream, the background O+ is
expected to be observed almost immediately upon exiting
the ionotail. In this case, the ion gyrocenters are a
gyroradius above the exobase. Altogether, at intermediate
distances from Titan, finite gyroradius effects are important, while very near the ‘‘ionopause’’, where the flow
speeds are low, 10–15 km s1, the flow is more fluid like.
The CAPS data below support these interpretations.
TOF spectrograms in the mass loading region for four B
cycle time intervals (4 min) are shown in Figs. 7a, b, c
and d, starting at SCET’s: 15:02:00, 15:06:16, 15:10:32 and
15:14:48, respectively. These cycles were briefly discussed in
Paper IV and are shown here for completeness and with
improved color scales and greater TOF spans. For
reference, the Cassini magnetometer observed the beginning of magnetic field draping at 15:10 SCET (Backes
et al., 2005), occurring just before the third B cycle.
Spectral signatures of H+ and H+
along with the
2
accompanying H are identified. The ion energy ranges
in the first B cycle, Fig. 7a, are 0.13–3.4 keV for H+ and
0.3–1.2 keV for H+
2 . Considering the presence of a
neutral exospheric source for these ions, their spectra are a
mixture of ambient and pickup ions. In contrast, the
heavier ion spectra in the range of 14–16 amu are observed
in a narrower energy range of 3.4–4.0 keV. As discussed
below, narrow energy ranges or concomitant ion beams are
expected for the heavier pickup ions whose gyroradii are
much larger than the scale heights of their exospheric
source gasses. Of particular interest in this mass loading
region is the missing ambient O+ fingerprint, O, which is
consistent with the collisional removal of O+ by Titan’s
atmosphere on those magnetosphere plasma flux tubes
inside about a gyrodiameter of the Titan’s exobase. The
concept of removing ambient magnetospheric ions was
discussed in Voyager 1 Papers I and II. More recently, it is
shown in Paper IV that O, and therefore O+, was missing
from all B cycles starting at 14:53 SCET (just outside the
mass loading region) and extending inward to the B cycle
starting at 15:14:48 SCET (just outside the ionopause).
The outer distance of this ambient O+ ‘‘clearing region’’ is
12,500 km, as measured along the –y-axis (Fig. 1a). The
velocity distribution of the ambient O+ ions are thought to
be in a shell velocity distribution. This has been shown to
be the case by Szego et al. (2005) through analysis of the
16 amu magnetospheric ions observed by the IMS. Using
an MHD model, Ledvina et al. (2005) demonstrated that
the ambient heavy ions could be described in terms of a
shell distribution. In the clearing region, O+ has fallen
below the detection level of the IMS; however, it must be
present at some lower level because some of the O+ orbits
are in transit to the atmosphere before they are absorbed.
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Fig. 7. IMS coincident TOF color spectrogram shown as energy per charge, E/Q (eV), vs. TOF channels with ion counts depicted by the color legend in
logarithmic counts to right. The spectrograms in (a) through (d) are single B cycles observed sequentially in the mass loading region preceding entry into
Titan’s ionosphere. The total number of counts for each B cycle is obtained by multiplying the numbers in the color legend by 8. The four B cycles (each
taking 4 min 16 s) start at SCET’s: 15:02:00, 15:06:16, 15:10:32 and 15:14:48, for spectrograms 6a through 6d, respectively.
In the future, it should be possible to estimate the densities
of these ‘‘ions in transit’’ using a suitable finite gyroradius
model. It follows that the atmosphere should similarly
remove any ambient large gyrodiamenter ions such as
+
N+/CH+
2 and CH4 .
As the above suggests, those ambient O+ ions in the
magnetospheric plasma flowing past Titan and also
penetrating its atmosphere are collisionally removed. In
the following, we expand upon a cursory discussion of the
removal rate presented in Paper IV. To estimate the
removal rate of the ambient O+ ions, we assume the
exobase is the highest altitude where collisions are
important. Collision rates increase exponentially over
relatively thin layers below the exobase because the
atmospheric scale height (80 km, Table 1) is much less
than the exobase radius 4000 km. These layers are also
thin with respect to an ambient O+ ion’s gyroradius of
4250. The observed outer boundary of the O+ clearing
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Table 1
Ion gyroradius, rg, and neutral gas scale height at exobase, H, are in km
Pickup ion length
H+
N+
CH+
4
N+
2
rg
H
rg/(H)
191
2185
0.087
2680
156
17.2
3060
137
22.4
5360
78
68.7
The parameters used to obtain these values here and throughout are the
following: background speed, Vb ¼ 110 km s1; magnetic field, B ¼ 6 nT
(Backes et al., 2005); exobase temperature T ¼ 149 K (Waite et al., 2005).
region of 12,500 km can be used to estimate the ambient
O+ ion gyrodiameter by subtracting the exobase radius
of 4000 km to obtain a gyrodiameter of 8500 km or
a gyroradius of 4250 km with a guiding center at y ¼
8250 km. This empirical estimate of the ambient gyrodiameter is considered a lower value because the more
energetic O+ ions in the velocity distribution that are
removed from the flow would have larger gyrodiameters.
Because the ions are in a shell distribution, the corresponding depth of penetration in the collisional region should be
thin relative to that of broader Maxwellian distributions.
The ambient O+ ions that penetrate the atmosphere
move toward Titan from above and below as they travel
along magnetic flux tubes flowing past Titan. The ions
must also move in the x-direction on guiding centers
bounded by |y|o8250 km. This outer boundary is outside
the distance where any noticeable deflection of magnetospheric flow around Titan has been observed [e.g., the first
indication of field line draping occurred at 15:10 SCET
(Backes et al., 2005) and slowing down begins after
Fig. 7a)]. As the ambient ions move along a magnetic flux
tube toward Titan, they have a velocity component along
the magnetic field and one in the rotational direction
(+x-direction of Fig. 1a). The guiding center velocity in
the x-direction is the rotational flow speed of 110 km s1.
We assume that about half of the ions above the equatorial plane move downward with speeds near the thermal
speed. A conservative estimate for this speed is the bulk
flow speed, 110 km s1, since the background plasma
is subsonic. Combining the components in the x- and
y-directions, the total speed
pffiffiffiffiffiffiffiffiof an ion moving toward
Titan would then be 3=2 110 km s1 ¼ 135 km s1 ,
where a pitch angle of 451 was assumed. In this case, the
downward flux of ambient O+ ions is 4 105 cm2 s1,
using half of the observed density (Paper III) of 0.06 cm3.
The outer boundary of the effective area where collisions
with the atmosphere can occur is approximated by the
circular cross section of radius 8250 km, the maximum
guiding center position of an O+ ion reaching the exobase.
Thus, the effective area or clearing area is 1.2 1018 cm2.
The net loss rate of ambient O+ is then 9.5 1023 s1,
accounting for the fluxes from above and below. This value
needs to be adjusted downward because a fraction of the
ions in the flow do not reach the exosphere because the ion
gyration time is greater than the time it takes a magnetic
flux tube to flow across the collision area. It can be shown
1219
that the average time collisions are occuring in the collision
region (time is larger near the equatorial region and
vanishes at the poles) as a magnetic flux tube flows through
the region is otc4104 s. Since the gyroperiod of an
ambient O+ ion is tg175 s, the effective fraction of the
density that actually collides with the atmosphere is 104/
175 ¼ 0.59, yielding a net loss rate of 5.6 1023 s1. This
value is about 37% less than the one obtained in Paper IV,
which used a somewhat different approach and did not
correct for the average collision times. Because of the
complexity of the interaction region, more accurate loss
rates will require a 3D kinetic model or its equivalent.
Furthermore, because of the temporal variability of the
environment around Titan, more ion measurements are
needed to obtain an improved picture of the O+ clearing
region.
4. Pickup ions and their distributions
As described in Paper I, the newly born pickup ions are
accelerated by the motional electric field, E ¼ V B,
pointing away from Titan when in Saturn’s equatorial
plane, where V is the background velocity of Saturn’s
rotating magnetosphere and B is its magnetic field. In the
region outside magnetic field draping, the ions move in a
plane perpendicular to B, which is also perpendicular to the
plane containing both V and B. Their distribution in
velocity space is two-dimensional and referred to as a ring
distribution. At distances well away from Titan, newly
born pickup ions move in cycloidal trajectories in planes
approximately parallel to Saturn’s equatorial plane (ions
born at essentialy zero velocity relative to background
speed). As the magnetic field begins to drape around Titan,
these planes begin to tilt from the equatorial plane. The
field was observed to start draping at 15:10 SCET
(Backes et al., 2005), just before the B cycle of 7c. The
draping becomes more prominent during the B cycle of 7d.
Thus, the ions traveling between the third and fourth B
cycles will move on surfaces out of the equatorial plane,
which tend to be perpendicular to the magnetic field. In
particular, newly born ions move perpendicular to the
instantaneous plane containing both V and B, while
previously born, energetic ions tend to stay closer to their
plane of origin.
Inside the heavy ion background erosion region, we
identify the remaining 14–16 amu ions as pickup ions,
where the 16 amu ion is identified as CH+
4 since methane is
the dominant 16 amu exospheric constituent (Paper II) as
seen in Fig. 6. The mass 14 amu pickup ion could be a
mixture of N+ and CH+
2 , where the former is the dominant
14 amu exospheric gas and the latter ion is a fragment of its
+
parents CH4 and CH+
4 . Another 28 amu ion, HCNH ,
identified as a possible dominant ionospheric ion in Paper
I, and confirmed by the Cassini INMS (Kasprzak, private
comm.), has also been suggested as a possible ion in the
mass loading region. If this is the case, it implies that some
of the plasma in Fig. 7d is of ionospheric origin. These ions
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would then have been scavenged by the interaction of the
externally flowing plasma with the ionosphere, a wellknown process observed at Venus.
Light constituent pickup ions such as H+ and H+
2 can be
identified by the abrupt drop in the energy distributions at
the pickup ion cutoff energies (maximum possible energy at
maximum ion speed of 2 the ambient plasma flow speed)
However, when the ion gyroradii exceed the scale heights
of their neutral source gasses, the ions can no longer be
identified by such cutoff energies. This is demonstrated by
the ion flux distributions shown in Fig. 8, which are plots
of analytic solutions to the Vlasov equation (Hartle and
Sittler, 2004). This analysis clearly shows that the ratio of a
pickup ion’s gyroradius, rg, to its source scale height, H, is
a fundamental parameter in determining the nature of its
velocity distribution as can be realized from the corresponding ratios in Table 1. The figure shows pickup ion
flux distributions for exponential neutral exosphere sources
that increase along the x-axis. A uniform magnetic field is
in the z-direction (see inset), the background velocity is in
the x-direction and the motional electric field is in the
y-direction. The distribution functions, f, have been
normalized by 2rgRN0/V3b, the product of their respective
gyroradii, ion production rates and neutral densities (at the
observation point) divided by the cube of the background
speed. The component velocities are normalized by the
background speed, Vb. The velocity space geometry defines
the total velocity, V, and the corresponding velocity, v, in
the moving frame at an angle y to Vx (i.e., x-axis along
direction of ambient flow). The gyroangle y has the range
–p to p, where pickup ions are born with vanishing velocity
at y ¼ p, attain their maximum velocity at y ¼ 0 and return
to zero velocity at y ¼ p. In the source region, the flux
distributions of the light species are relatively strong up to
their cutoff energies while the heavy constituent fluxes,
with rg =H 1, are only strong at low total velocities and y
Fig. 8. Flux distributions Vf vs. y (radians), where f is normalized by
2rgRN0/V3b. Velocity space geometry is shown in the inset and described in
the text. The pickup ion gyroangle y range is –p to p.
near p. The latter result indicates that the heavy ions have
only traveled a short distance from birth xH rg , up to
about a scale height away. When away from field line
variations or draping, the beam like nature of the heavy
ions in the source region continues downstream, beyond
the source, with higher energy flux peaks distributed on
cycloidal trajectories.
To further illustrate why an ions velocity distribution is
dependent on the ratio of its gyroradius to the source gas
scale height and why CH+
4 should be observed as a narrow
beam, consider the CH+
4 pickup ion trajectories shown in
Fig. 9. The cycloidal pickup ion trajectories are drawn to
scale (using the parameters of Table 1), where the ions are
born at zero velocity on the line of origins (see Hartle and
Killen, 2006), assuring that they all pass through the
observation point, O, on the TA trajectory. This approximate example assumes that the pickup ion trajectories are
shown moving at sufficient distances outside the interaction region where the background plasma is flowing
uniformly. The outer boundary of the interaction region
is approximated by the point on TA where magnetic field
draping is first observed at 15:10 SCET (Backes et al.,
2005). Although the cyan curve in Fig. 9 passes through
this point, the projection of points beyond it is only
intended to illustrate that there is some boundary beyond
which the background plasma flows uniformly. Similarly,
an approximation to the ionopause is also shown, where it
is drawn to pass through the ionopause at r 4600 km
(estimated in Paper II) at the subflow point and also the
Fig. 9. CH+
4 pickup trajectories (black) born with zero velocities on the
line of origins (red) that assures they pass through the observation point,
O, on the Cassini TA trajectory (magenta). Titan and the exobase are
shown in black while ionopause and magnetic draping boundaries are
sketched in green and cyan, respectively. The blue lines correspond to
decadal variations of CH4, where the innermost level is 103 cm2 from
model of Sittler et al. (2005).
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point on TA where cool ionospheric electrons are first
observed (Coates et al., 2005). The blue lines correspond to
decadal variations of CH4, where the innermost level is
103 cm2 from the model of Paper II. We note that the
outer boundaries of the mass loading region (background
slowing down region) and the field draping region begin on
about the same boundaries. Because the background
plasma inside these boundaries moves more slowly than
outside, the ions born within these boundaries will not be
accelerated as fast as in the uniform flow region outside
and consequently will not reach as far as the observation
point, O. In this case, the pickup ions born just outside
about the first scale height beyond the slowing down region
will arrive at the observation point with the largest fluxes,
since they come from points with the largest CH4 densities
and CH+
production rates. The energy of an ion
4
accelerated in the motional electric field over a distance L
(103 km) in the –x-direction from birth place to the
observation point, O, will have an energy 0.66L (kV).
The CH+
4 energies observed in the region surrounding O
are similar to those in Figs. 7a and b. Therefore, if the peak
energy is 3 kV, then L4.5 103 km. This distance traced
backward on a trajectory from O corresponds to a position
x8 103 km and y2.5 103 km on the line of
origins. This trajectory is identified by a red X on the line
of origins in Fig. 9. In this case, the bulk of the ions
observed at O are likely to have been born along the line
origins from X outward to the point where CH4 has
decayed one or two scale heights. Referring to Fig. 6, the
scale height of CH4 is 700 km at point X. The energy
variation of ions at O born over one or two scale heights
near X is 0.5–1 kV. This narrow energy range is similar to
the peak widths of the CH+
4 pickup ions in Figs. 7a and b.
This semi-quantitative example is only intended only to
illustrate that when the gyroradius exceeds the ion source
scale height, the resulting energy distribution is a narrow
beam. Further data analysis and modeling are needed to
quantify this picture.
As mentioned above, newly born ions move perpendicular to the instantaneous plane perpendicular to the V–B
plane, while previously born, energetic ions tend to stay
closer to their plane of origin. In the pickup region, the
+
+
dominant heavy ions, N+/CH+
2 , CH4 and N2 , have values
of rg =H 1 (Table 1). Consequently, considering the
above, the observed distributions will span a narrow
velocity range, with peaks populated with ions born over
the range of an atmospheric scale height. Similarly, upon
entering the field line draping region, some additional
spread in the ion beam will occur; however, since
rg =H 1, the space and energy spread will not be large
because the ions were born over a length scale H, relative
to their gyroradii, rg. The spread will only become large
when the scale length for magnetic field variations is much
less than the atmospheric scale height, which is not the case
in the ion pickup region (see Backes et al., 2005).
We use the actuator motion of the IMS in the mass
loading region to aid in identifying pickup ion distribu-
1221
tions. As the actuator continuously sweeps about 1001
above and below the equatorial plane, the collimator
should accept pickup ions over a segment of its angular
sweep. If the ions were not confined to the narrow beams
described above, they would be observed over wider
angular swaths of the collimator. The spectra of the
14–16 amu ions in Fig. 7 appear to be consistent with the
signatures of pickup ions in narrow beam distributions.
Furthermore, the pickup ion energies ‘‘jump’’ to lower
energy ranges on passing from Figs. 7a–d, corresponding
to four separate B cycles. Such jumps are expected for a
beam-like distribution because the collimator (see collimator angles in Fig. 10 below) makes only one complete pass
during each B cycle, assuring the likelihood that an ion
beam would be crossed. The energy jump occurs because
the plasma speed continuously decreased during each B
cycle while the distribution is only observed during a short
portion of the cycle when the collimator can accept ions in
the beam, which is a point where the background plasma
speed is less than that of the preceding B cycle. Other
distributions, such as a shell distribution, would have
broader velocity spectra observed over larger collimator
scan angles, permitting the effects of more plasma
deceleration to be included.
5. Mass loading
Mass loading was qualitatively identified in the singles
data of Fig. 4 and made more realistic with the identification of pickup ions through analysis of the TOF data in
Fig. 7, which can also be used quantitatively to study the
slowing down properties. For example, passing from
Figs. 7a–c, there is a gradual decrease in the ion energies
of about 1.5 to 2 times in the 14–16 amu range,
respectively. However, as Cassini moved even closer to
Titan, passing from Fig. 7c, d, there is a significantly
greater energy drop of 125 times in these ions. Such a large
energy decrease can be attributed to at least two factors, an
increase in the density (or production rate of the ions) and
mass of the pickup ions born upstream. Referring to the
exosphere profiles of Fig. 6, one can see that this is likely to
be the case; i.e., there is a rapid increase in the neutral
source gas densities as the distance to Titan decreases.
Furthermore, the rapid increase is due to the heavier
constituents, CH4 and N2, where the latter (with shorter
scale height 85 km) becomes dominant as the exobase is
approached, which is consistent with Fig. 6 and confirmed
by the recent INMS measurements (Waite et al., 2005)
described above. All of this is consistent with our initial
identification of the 28 amu ion as N+
2 , with flow speeds
10–15 km s1, and its appearance in the last B cycle
observation, which is just before entering the cooler, denser
ionospheric plasma observed by Coates et al. (2005),
bounded by an ionopause-like boundary. This also
supports the mass loading picture inferred from the PLS
observations on Voyager 1 (Paper I), where N+
2 was also
thought to be the heavy mass loading ion producing very
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R.E. Hartle et al. / Planetary and Space Science 54 (2006) 1211–1224
Fig. 10. Electron properties from ELS observations. Top panel: electron energy vs. SCET vs. count rate (color scale in logarithmic counts to right); second
from top: electron density, ne, vs. SCET; third from top: electron temperature, Te, vs. SCET; fourth from top: actuator scan angle above and below
equatorial plane at 01.
low speeds just outside the ionopause. Furthermore, the
model calculation in Paper II obtained flow speeds of the
background plasma that also supported the mass loading
picture described in Paper 1 and above.
6. Electron measurements
Electron energy count rates, densities, temperatures and
scan angles obtained from the Cassini ELS are shown as a
function of SCET in Fig. 10. The densities and temperatures have a number of similarities to those observed by
PLS on Voyager 1 (Paper I), as shown in Fig. 11a. For
example, both show a buildup of density as Titan is
approached. On entering the ionosphere on the inbound
leg, ELS densities continue to increase, reaching a peak in
the ionosphere. No such peak appeared in the PLS
measurements because Voyager 1 was in the magnetotail
(we refer to an ionotail as that part of the tail within the
ionosphere while further downstream we call it a magnetotail), far from the ionosphere. On the inbound and
outbound legs of Voyager 1 and Cassini, but well beyond
the ionotail, both ELS- and PLS-derived electron densities
are similar at 0.1 cm3. The corresponding ELS electron
temperatures ranged between 100 and 1000 eV, being
higher than the 200 eV observed by PLS. The electron
velocity distributions observed by the Voyager 1 PLS in the
wake region (Paper I) are shown in Fig. 11b. A unique
feature of these distributions is the electron depletion or
‘‘bite-out’’ in the electron spectra above 700 eV. The
electron bite-out of energetic magnetospheric electrons
observed by the PLS is also seen in the flux spectrum of the
ELS. The bite-out was interpreted in Paper I to be due to
Fig. 11. (a) Voyager 1 electron densities and temperatures vs. SCET.
(b) Electron distribution function vs. electron speed vs. SCET.
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scattering down in energy of Saturn’s magnetospheric
electrons by Titan’s neutral atmosphere through which the
draped magnetic field lines thread. Electron cooling on
Voyager 1’s inbound approach to the magnetotail is
consistent with the addition of cool photoelectrons during
pickup ion formation. Such cooling is observed by ELS as
Cassini moved into the more dense ionosphere. The
resulting density buildup is consistent with the addition
of pickup ions and the corresponding slowing down due to
mass loading. These features are absent on Voyager 1 and
Cassini as they exit the ionosphere on their outbound legs,
which is consistent with the decreased formation of pickup
ions on the Saturn facing side of Titan. We note that the
Voyager 1 electron spectrum was reproduced rather well by
the model of Gan et al. (1992) and should be applicable to
the Cassini measurements due to the similarities in the PLS
and ELS observations.
7. Summary
The fortuitous similarities of the plasma wake trajectories of the Cassini TA and Voyager 1 flybys, occurring
near Saturn’s local noon, made it possible to make direct,
meaningful comparisons of the plasma observations made
by CAPS and PLS. In general, the features of the
interaction of Saturn’s rotating magnetosphere with
Titan’s atmosphere observed by the two instruments were
found to be similar. In each case, the background
magnetospheric plasma in the vicinity of Titan was
observed to be subsonic, where a speed of 110 km s1
(subrotation) was inferred from CAPS measurements. On
the anti-Saturn side, the slowing down of magnetospheric
plasma observed by the PLS was also found to be the case
for the CAPS measurements. Each instrument observed the
plasma to slow down at an increasing rate as the ionopause
was approached. During the Cassini TA flyby, the plasma
slowed down from 110 to 10–15 km s1. In Paper I, the
slowing down was attributed to mass loading by pickup
ions born from the extended exosphere. The principal
+
+
+
pickup ions, H+, H+
2 , N , CH4 and N2 were inferred
from the PLS measurements (Papers I and II) and
confirmed by CAPS observations. The latter observations
made ion identification more definitive due to the TOF
capability as well as having higher data rates than Voyager
+
1. The heaviest of the pickup ions, CH+
4 and N2 , appear to
do most of the mass loading, as suggested in the model
of Paper II. The B cycle, E/Q vs. TOF, measurements of
Fig. 7 showed that the heavy pickup ion distributions can
be interpreted in terms of narrow energy ion beams as
expected from ions whose gyroradii are much larger than
the source gas scale height. Although the 28 amu pickup
ion has been identified as N+
2 , one might question the
possible presence of the ionospheric ion, HCNH+, which
may have penetrated the ionopause and joined the downstream flow of pickup ions. Additional flyby measurements
and further analysis of the fragments in the IMS may shed
light on this question.
1223
Ambient magnetosphere O+ was observed to be
removed from the flow by the IMS on CAPS. Such a
clearing zone for the ambient magnetosphere was initially
identified using Voyager 1 observations, where magnetospheric ions such as O+ are collisionally removed by
Titan’s upper atmosphere. The dimension of the O+
clearing area is about an ion gyrodiameter, 8500 km,
above the exobase, as measured in the ion’s plane of
gyration. This oxygen source (water ions may also be
present) could be an important source of oxygen–water to
Titan’s atmosphere. The oxygen could be locked up in the
reducing atmosphere of Titan in the form of CO (Lutz
et al., 1983), CO2 (Samuelson et al., 1983) and water vapor
(Coustenis et al., 1998). Another important source of water
and oxygen to Titan’s atmosphere is micrometeorites
(Samuelson et al., 1983]). But, the CO in Titan’s atmosphere could also be primordial (Owen, 1982). As the
properties of the ambient magnetosphere change, the
characteristics of the clearing area will change. Observations from future flybys may make it possible to obtain
parametric relationships between the two regimes.
A number of similarities were found in the PLS and ELS
electron measurements during their flybys of Titan. For
example, well outside the ionotail, the electron densities
were observed to be similar at 0.1 cm3, while the PLS
electron temperature of 200 eV was within the broader
range, 100 to 1000 eV, observed by the ELS. Furthermore, both instruments observed a buildup of electron
density as Titan is approached. Both Voyager 1 and
Cassini electron instruments observed the presence of a
high-energy electron bite-out inside the ionopause boundary. The bite-out is due to the scattering down in energy of
magnetospheric electrons, on draped field lines, as they
collide with Titan’s upper atmosphere, leaving behind
photoelectrons and secondary electrons produced at
energies predominantly less than 10 eV. When future
flybys cut through the magnetotail at a distance from Titan
similar to that of Voyager 1, meaningful comparisons of
ELS and PLS electron properties should be possible. In this
case, the position of Titan should be close to Saturn noon,
as with the Cassini TA flyby.
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